Zebra mussels have spread to more than 250 Wisconsin lakes and rivers since being first being identified in 1986. These small mollusks colonize docks, floats and boating equipment; their sharp shells can injure swimmers. They form dense colonies, up to 65,000 individuals per square foot, that can interfere with water supplies, disrupt food webs and alter ecosystems.

Like many other recent Great Lakes invaders, the zebra mussel is native to the Baltic region of Europe and was carried to our shores in the ballast water of ocean-going freighters. Zebra mussels were originally introduced to Lake St. Claire in the mid 1980’s. Since then, zebra mussels have spread throughout the Great Lakes region, into the Mississippi River, down to the Gulf of Mexico and into Canadian waters. They are spreading to inland lakes in many states including Wisconsin.

How Do They Spread?

Zebra mussels are prolific breeders; a single female can produce tens of thousands of eggs annually. Eggs and sperm are released into the water column. After fertilization, the microscopic larvae, called veligers, float about in the water column for up to a month. At that time, the larval mussel transforms into a juvenile and settles to the bottom. Once settled, it adheres to a hard surface by means of sticky fibers called byssal threads.

Zebra mussels are able to spread easily due to the planktonic larval form and the adhesive adult form. The larvae can be carried in bilge water, live wells, bait buckets, or anything else that holds water such as dive gear. The juveniles and adults can attach to any hard surface that is submerged for a prolonged period of time including boat hulls, floats, anchors and lines, piers, swimming platforms, rocks, wood and vegetation. Adults are easily seen with the unaided eye; juveniles feel like sand or grit on the surface of boat hulls, motor or other submerged parts. Larval mussels can only be seen with a microscope.

The larvae need to remain wet to survive, but adults and juveniles can live out of the water for several days. A boat carrying adult or juvenile zebra mussels on the hull, larval zebra mussels in the bilge, live well water or plants tangled on the trailer can easily introduce zebra mussels from one lake to another.

Environmental Effects

Zebra mussels are filter feeders; they filter planktonic food from the water around them. Each zebra mussel can filter a liter (over a quart) of water per day. As they feed, they remove plant and animal plankton from the water. These tiny animals form the first food for the larvae of many desirable fish and are the food for other native species. As zebra mussels take plankton from the water, they compete with our native species for this food. The juveniles and adults can attach to any hard surface that is submerged for a prolonged period of time. Zebra mussels also make the water clearer. Though we often think of clear water as better water, clear water means there is less food for our native plankton-feeding organisms. Clear water also makes it possible for submergent vegetation to grow in areas that were previously too turbid to allow plant growth. This can contribute to weed-choked shorelines and changes in the food web and fish populations.

Zebra mussels adhere to the exterior of a clam.

Preventing the Spread

Lakes are usually identified as being colonized only after the mussels have been present for some time, perhaps years. People using boats, personal watercraft and diving in these and other waters may have unknowingly been vectors for the spread of zebra mussels into other lakes. Likewise, lakes not yet known to be colonized may already have zebra mussels in them. For this reason we need to take precautions to prevent the spread of zebra mussels and other invasive species every time we plan to move from one lake or river to another.

Remove plant parts from the intake and run personal watercraft and jet boat engines for a few seconds out of the water.

Let boats, trailers and equipment dry for 5 days before moving to new waters.

As you pull docks, swim floats, anchors, lines and boats from the water, look and feel for zebra mussels on surfaces that were submerged through the summer season. Juvenile mussels may not be easily seen, but will feel like grit on the surface. Larger mussels are easily seen and may occur in groups or as separate individuals. If you find zebra mussels in a lake not known to be infested, save some of the specimens and please contact Wisconsin DNR, UW Extension or Wisconsin Sea Grant at the numbers below.

Other Invaders

Zebra mussels are not the only invaders trying to find their way into our inland lakes and streams. Recently, round gobies were found in Little Lake Butte des Morts near Lake Winnebago; the spiny water flea, a large predacious species of zooplankton, has been found in a handful of lakes in northern and southern Wisconsin. If these and other species find their way into our favorite lakes they may forever alter the ecosystem as they already have in Lake Michigan.